Unveiling Low-Energy Emission in 2D Organic–Inorganic Perovskites: A Photorecycling and Electron–Phonon Coupling Study

Abstract

2D hybrid organic–inorganic perovskites have demonstrated outstanding optoelectronic properties. Among their photophysical features, low-energy photoluminescence (PL) peaks are often observed, yet their origin remains not fully understood, particularly due to the coexistence of multiple possible emission pathways. To address this, we hypothesized that structural rigidity─determined by the organic spacer─may influence photon recycling and, consequently, the nature of low-energy emission. We thus investigated the origin of low-energy emission in both (BA)₂PbBr₄ and (PEA)₂PbBr₄ across different material morphologies including microcrystals and colloidal nanoplatelets. Through a combination of structural characterization, steady-state and temperature-dependent PL measurements, and density functional theory (DFT) calculations, we demonstrate that the low-energy emission observed in microcrystalline samples arises predominantly from photon recycling rather than self-trapped excitons (STEs) or defect states. Importantly, we find that the size and orientation of the crystals strongly affect the observed PL, supporting the interpretation that total internal reflection (TIR) within the perovskite acts as a waveguiding mechanism for photon propagation and reabsorption. Our findings reveal that the structural rigidity of the perovskite plays a crucial role in governing photorecycling photon propagation. Specifically, the flexible lattice of (BA)₂PbBr₄ facilitates photon recycling with energy losses, resulting in pronounced low-energy emission, whereas the rigid structure of (PEA)₂PbBr₄ better preserves the photon energy during propagation. The observed differences in electron–phonon coupling further support the role of structural flexibility in modulating the transport of emitted photons. These insights provide a deeper understanding of light-matter interactions in 2D perovskites and highlight the importance of crystal engineering in optimizing their optical properties. By tailoring the organic spacer and controlling structural rigidity, it is possible to fine-tune emission characteristics for applications in optoelectronic devices.